Device for failure detection in series-connected semiconductor rectifiers

ABSTRACT

A device for failure detection in series-connected semiconductor rectifiers (1) comprises transformers (2) whose number is equal to that of the rectifiers (1) and which comprise input windings (3), supply windings (6), and additional parallel-connected windings (9), RC-circuits whose number is equal to that of the rectifiers (1), having resistors (4) and capacitors (5), and also a pulse generator (7), and a measuring element (8). 
     The input winding (3) of each transformer (2) has one lead-out thereof connected to a cathode of the respective rectifier (1), and another lead-out to the capacitor (5) of the respective RC-circuit. The resistors (4) of each RC-circuit are connected to the anodes of each rectifier (1). The supply windings (6) of the transformers (2) are connected to one another in series and connected by one terminal to the first lead-out of the pulse generator (7). The second lead-out of the pulse generator (7), together with the second terminal of the circuit comprising the series-connected supply windings (6), are connected to a measuring element (8).

TECHNICAL FIELD

This invention relates to power converters and, in particular, todevices for failure detection in series-connected semiconductorrectifiers.

BACKGROUND ART

Heavy-duty high-voltage semiconductor converters are becomingincreasingly popular in various fields of application in science andtechnology. They are used in AC electric drives with induction andsynchronous motors, in stabilized power supplies, in plasma-jetelectron-ion technology, in power supply systems for electrophysicalresearch, and other various fields.

Application of powerful high-voltage semiconductor converters in thesefields offers the advantages of improved technological processes,automation of such processes, efficient use of electric power,minimization of losses of this electric power.

One common feature of heavy-duty converters rated for 3-20 kV consistsin the use of series-connected semiconductor rectifiers. Suchsemiconductor rectifiers are extremely sensitive to overloads because oftheir small size, minor heat capacity, and insufficient reserve ofelectrical strength. Special techniques and means for protection ofsemiconductor rectifiers should be developed for efficienttroubleshooting as part of their maintenance.

The problem of developing failure detection devices has become a seriouschallenge and cannot be solved without making use of the repertoire ofmodern power converters. Such failure detection devices should becapable of preventing emergency processes in heavy-duty high-voltageconverters, minimizing the time needed for trouble hunting, providingfor preventative maintenance by few unskilled personnel, resulting inlower costs of maintenance.

Known in the art are devices for failure detection in series-connectedsemiconductor rectifiers (cf., for example, the USSR Inventor'sCertificate No. 481 963, Cl. H 02 H 7/10; the USSR Inventor'sCertificate No. 620 919, Cl. H 02 H 7/10) which comprise, in order todetermine if the rectifier is broken down or not, a measuring elementincorporated into the diagonal of an unbalanced bridge composed of acircuit of series-connected monitored rectifiers, each rectifier beingshunted by a resistor and a resistor voltage divider. These devicesrealize the so-called passive failure detection technique for monitoringthe series-connected semiconductor rectifiers. This technique is basedon detecting a signal produced in the bridge diagonal when the balanceof this bridge is upset by the breakdown of a rectifier. The accuracyand reliability of such devices are not satisfactory, since much dependson the operating conditions of the semiconductor rectifier and themagnitude of the supply voltage. Such devices, therefore, have to beindividually adjusted, and, since they realize the passive detectiontechnique, they cannot be used for failure detection in semiconductorrectifiers before the supply voltage is applied to these rectifiers. Inaddition, such testing devices have an extremely complicated maintenanceprocedure.

Also known in the art is a device (cf., for example, British Pat. No.1,540,300 Cl. H 02 M 1/18) which also realizes the passive testingmethod for failure detection in series-connected semiconductorrectifiers by measuring the forward and reverse voltages across theserectifiers. This device uses optoelectronic sensors whose number isequal to that of the series-connected semiconductor rectifiers. In thismanner the protection system is conductively isolated from the powerportion of the converter.

The disadvantages of this device consist in that it is too complicated,requires additional logical processing of information fed fromoptoelectronic sensors, and comprises a large number of elements locatednear the rectifiers, which impairs its reliability. Faulty rectifierscannot be detected before the supply voltage is applied.

Also known in the art is a device for controlling and failure detectionof series-connected rectifiers (cf., for example, the USSR Inventor'sCertificate No. 256,049, Cl. 21 d2 12/03 H 2M), comprising a pulsegenerator having the first lead thereof connected to a terminal of thecircuit composed of series-connected supply windings of currenttransformers, whose number is equal to that of the rectifiers, the inputwinding of each transformer being connected to a cathode of a respectiverectifier, and RC-circuits whose number is equal to that of therectifiers. The noise immunity of the device is improved by installingadditional transformers whose number is equal to that of the rectifiers,the cores of the first and additional transformers being arranged insymmetry to the series-connected supply windings.

This device can, in principle, provide failure detection in rectifiersirrespective of the magnitude of the supply voltage of the converter.

But the device is deficient in that the circuitry and design areovercomplicated. In addition, it is not sufficiently reliable because ofthe large number of series-connected elements.

DISCLOSURE OF THE INVENTION

The invention is to provide a reliable device for failure detection inseries-connected semiconductor rectifiers, which is capable of detectingfailures of rectifiers irrespective of the magnitude of the supplyvoltage and operational conditions of the converter by altering thecircuit of the transformer and inserting a measuring element.

This object is achieved in that in a device for failure detection inseries-connected semiconductor rectifiers, comprising a pulse generator,RC-circuits whose number is equal to that of the rectifiers and whichare connected by resistors to anodes of each rectifier, and transformerswhose number is equal to that of the rectifiers, the input winding ofeach transformer having one lead thereof connected to a cathode of arespective rectifier, while the series-connected supply windings of saidtransformer shave one terminal connected to a first lead of the pulsegenerator, according to the invention, transformers are provided withadditional parallel-connected windings, and second leads of each inputwinding of the transformer are connected to capacitors of respectiveRC-circuits, while a measuring element is connected to a second lead ofthe pulse generator and to a second terminal of the circuit composed ofthe series-connected supply windings of the transformers.

The device for failure detection in semiconductor rectifiers can operateirrespective of the magnitude of the supply voltage and the converteroperating conditions. This device can be used in different converters,makes them more reliable and their maintenance simpler, since faults canbe detected even before supply voltage is applied to the converter.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail, by way of example,with reference to the accompanying drawing which shows a device forfailure detection in series-connected semiconductor rectifiers,according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A device for failure detection in series-connected semiconductorrectifiers 1 (the sole FIGURE), according to the invention, comprisestransformers 2 whose number is equal to that of the series-connectedrectifiers 1. Input windings 3 of each transformer 2 are connected, viaresistors 4 and capacitors 5 of RC-cicuits, to the series-connectedsemiconductor rectifiers 1. Supply windings 6 of the transformers 2 areplaced in series-aiding connection and coupled to a pulse generator 7via a measuring element 8. Each transformer 2 is provided with anadditional winding 9, and these windings 9 of all transformers areconnected to one another in parallel. The input windings 3 of half thetransformers 2 are connected to the semiconductor rectifiers 1 byterminals having one polarity, the input windings 3 of the other half ofthe transformers 2 being connected by terminals of the oppositepolarity.

For clarity windings 3, 6 and 9 are shown as single-turn windings.

The device for failure detection in series-connected semiconductorrectifiers operates as follows.

The high frequency AC voltage fed from the pulse generator 7 is applied,via the measuring element 8, to the series-connected supply windings 6of the transformers 2. When all rectifiers 1 are fault-free anddeenergized, the circuits of the input windings 3 are open, and alltransformers 2 operate under no-lead conditions. Current consumed by thesupply windings 6, supplied by the pulse generator 7, and flowingthrough the measuring element 8 is, therefore, minimal.

When any of the series-connected rectifiers 1 is broken down, the inputwinding 3 of the respective transformer 2 becomes closed on the resistor4, since the resistance of the capacitor 5 at the voltage frequency ofthe pulse generator 7 is substantially less than that of the resistor 4.

Transformers 2 are magnetically coupled due to the parallel connectionof the additional windings 9, and can be thought of as a singletransformer having several windings. In case of a breakdown of anyrectifier 1, the load of the input winding 3 of the respectivetransformer 2, which is the resistor 4, may be regarded connected to thepulse generator 7 via the measuring element 8, taking into account thetransformation ratio of the input winding 3 and supply windings 6. Inthis situation, current drawn from the pulse generator 7 increases.Increased current makes the measuring element 8 operate.

The breakdown of two or more rectifiers 1 will result in furtherstepwise increase in current. In this manner, a multilevel failuredetection system for semiconductor rectifiers can be organized.

The resistance of the capacitor 5 at the frequency of the power supplyvoltage applied to the series-connected rectifiers 1 of the converter,when the rectifiers are energized, proves to be high in relation to itsresistance at the frequency of the pulse generator 7. In this manner thedevice for failure detection in series-connected semiconductorrectifiers is isolated from the converter operation and its power supplyvoltage. In addition, the interference of the switching currents of thecapacitors 5 of RC-circuits, which are the result of the power supplyvoltage, and the interference of the switching processes neutralize eachother because the input windings 3 of one half of the transformers 2 areconnected in opposition to the other half. The pulse generator 7 can bean RC-generator using, for example, either the Meissner or Hartleycircuits. The measuring element 8 is a comparator using, for example, astandard operational amplifier.

To summarize, the device according to the invention offers the followingadvantages. It permits reliable failure detection in rectifier circuitsirrespective of the operational conditions of the converter and themagnitude of its power supply voltage. The converter becomes morereliable, and its maintenance becomes much easier because failures ofthe rectifiers can be detected even before the supply voltage is appliedto the converter.

It can be safely assumed that this device for rectifier failuredetection, which possesses serious advantages and is uncomplicated indesign, can find many applications in thyristor converters rated for3-20 kV wherein thyristors are series-connected.

The preventive failure detection realized by the device, whereby faultyrectifiers can be detected before the converter is put into operation,combined with the simpler maintenance procedure, can become decisivefactors for using the device for protection and diagnostics of suchconverters as, for example, heavy-duty high-voltage thyristor frequencyconverters for starting synchronous motors (at hydroelectric pumpedstorage power plants, gas-turbine plants, stove blowers, etc.),high-voltage rectifier drives of pumps, fans and compressors in powergeneration industry, chemical industry, stabilized controlled rectifiersfor radio engineering and plasma chemistry, and many other types ofconverters whose failure because of poor failure detection techniquescan result in serious accidents and involve dramatic losses.

Industrial Applicability

The invention can be used for protection of semiconductor converters ofhigh power, such as controlled rectifiers, inverters, and frequencyconverters, rated for 3-20 kV, in which semiconductor rectifiers areconnected in series, against internal short circuits when some of theseries-connected semiconductor rectifiers are broken down.

The invention can also be used in diagnostic systems of such converters.

We claim:
 1. A device for failure detection in series-connectedsemiconductor rectifiers, comprising a pulse generator, RC-circuitswhose number is equal to that of said rectifiers and which are connectedby resistors to anodes of each rectifier, and transformers whose numberis equal to that of the rectifiers, the input winding of eachtransformer being connected by one lead thereof to a cathode of arespective rectifier, while the series-connected supply windings of saidtransformers are connected by one terminal to a first output of thepulse generator, characterized in that the transformers (2) are providedwith additional parallel-connected windings (9), and second leads ofeach input winding (3) of the transformers (2) are connected tocapacitors (5) of respective RC-circuits, a measuring element (8) beingconnected to a second lead of the pulse generator (7) and to a secondterminal of the circuit composed of the series-connected supply windings(6) of the transformers (2).